When generating changelogs for a range of commits, any commits both
added and reverted in that range are ignored. This works by looking for
commits with the pattern "This reverts commit X", then ignoring the
commits with the mentioned SHA.

Because commits are retrieved in reverse order (= newest first), and
revert commits always come after the commit they revert, we can keep
processing commits in batches; instead of having to first load all of
them in memory. This means the number of Gitaly calls remains the same.

As part of these changes, CommitsWithTrailerFinder is renamed to
ChangelogCommitsFinder, as its purpose is now more specific to the
process of generating changelogs; instead of being a generic "give me
commits with trailer X" finder.

See https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1583 for
more information.